Toxic exhaust gases and nitrogen oxides (NOx) occur in the context of the combustion process in diesel engines. To eliminate or break down these nitrogen oxides, it is known to inject a urea solution, by means of a metering pump, into the previously purified exhaust gas stream. The ammonia that is thereby released converts up to 80% of the nitrogen oxides into harmless nitrogen and water in a downstream SCR catalytic converter.
Because a urea solution is a chemically aggressive and very low-viscosity medium that has a tendency to crystallize, special pumps, in which the urea solution does not come into contact with the drive equipment of the metering pump, are used to deliver it. The delivery space is separated from the equipment space by, for example, a membrane or another flexible part.
The pump runs constantly during vehicle operation, establishing a pressure of, for example, 5 bar. Urea is present in the lines and systems. If the ambient temperature drops below the freezing point after the vehicle is shut off, the system would completely freeze up. Since not all components can withstand freezing, the urea solution must be pumped back into a reservoir container after the vehicle is shut off. In known systems, this occurs by means of a 4/2-way valve that reverses the delivery direction.